Audio recording devices become more and more important. Particularly, an increasing number of users buy communication devices, headphone-based audio recorders and mobile computing systems.
A microphone may be denoted as an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. Microphones are employed in many applications such as telephones, tape recorders, hearing aids, motion picture production, live and recorded audio engineering, in radio and television broadcasting and in computers for recording voice, and for non-acoustic purposes such as ultrasonic checking.
A common design uses a thin membrane that vibrates in response to sound pressure. This movement is subsequently translated into an electrical signal. Conventional microphones for audio use electromagnetic generation (dynamic microphones), capacitance change (condenser microphones) or piezoelectric generation to produce the signal from mechanical vibration.
JP S60-018100A discloses to detect directly a density change of a propagation medium by using a means that irradiates a laser light into the propagation medium of sound waves and a laser light detecting means to constitute a microphone. A laser beam delivered from a laser light emitting means is divided into two paths by a beam splitter. The first beam propagates through a solid medium; while the second beam propagates through a propagation medium of sound waves. Both beams are synthesized by a reflector and a beam splitter, and the intensity of the synthetic beam is detected by a photo detecting means. A density change of the medium is produced in response to the variation of sound pressure. This produces the variation of propagating speed of the beam and then the variation of phase. Then the sound pressure is detected in the form of a change of intensity of the synthetic beam by having the systemization and interference between the beams.
U.S. Pat. No. 6,590,661 discloses methods for remotely sensing sound waves in an optically transparent or semitransparent medium through detecting changes in the optical properties of the medium, that are caused by the sound waves. For example, to implement a microphone that can sense sound at a distance from the sound source. The variations in the attenuation or the phase of a beam of light that is received after passing through the sound waves are sensed and converted to an electrical or other signal. For the attenuation method, the wavelength of the beam of light sensed is selected to be one that is highly attenuated by a constituent of the medium, so that the changing instantaneous pressure of the medium due to the sound pressure waves can be detected through the changing light attenuation due to the changing density of the air along the light path. For the phase shift method, the velocity of light, and therefore its phase is changed by the changing density of the air due to the sound waves, and this can be detected through interferometric means.
JP H5-227597A discloses that, to obtain a microphone of low distortion, a broad band and a broad dynamic range, a diaphragm receiving an acoustic wave is excluded. An acoustic wave is inputted into an acoustic input part that is released and has air in it. A laser beam moves back and forth between a pair of reflecting mirrors to be one component of a Fabry-Perot laser beam interferometer so as to cross the acoustic wave. The optical path length of the laser beam is optically and equivalently changed with the change of the roughness/fineness of air corresponding to the acoustic pressure of the acoustic wave at each time, and a light receiving device catches it.
However, conventional optical microphones may still lack accuracy in detecting acoustic signals, particularly when environmental conditions change.